M. Wołcyrz

Effect of chlorine on microstructure and activity of Pd/CeO2 catalysts

The evolution of microstructure and activity in benzene hydrogenation of Pd/CeO2 catalysts, prepared from Pd chloride or Pd nitrate, upon reduction at 573–973 K has been studied by X-ray diffraction (XRD), selected-area electron diffraction (SAD), high-resolution transmission electron microscopy (HRTEM) and gas chromatography. The crucial role of chloride from the metal precursor in structural transformations of the ceria support has been established and attributed to its ability to form stable cerium oxychloride at temperatures as low as 573 K. A. rapid decline to zero of the activity of Pd/CeO2 catalysts in benzene hydrogenation was observed when the temperature of reduction was increased from 573 to 773 K. This effect could not be explained by sintering, but XRD data indicated that a metal–support reaction leading to Pd–Ce alloy formation was likely to be the cause. At higher temperature (873 K) the coverage of Pd particles by a thin overlayer (probably CeOx) was observed by HRTEM.

Microstructure of Pd/CeO2 catalyst: Effect of high temperature reduction in hydrogen

Abstract The effect of high temperature reduction (HTR) in hydrogen (up to 1180 K) on the microstructure of 9 wt.-% Pd CeO 2 catalyst was studied by HRTEM and XRD methods. Reduction of the catalyst at or above 973 K caused severe recrystallization of CeO2 and Pd with simultaneous strong interaction between the two components appearing as three phenomena: epitaxial growth of small Pd particles on CeO2 (most frequently with [111]Pd∥[111]CeO2); decoration of large Pd particles with ordered CeO2 overlayer and expansion of the lattice parameter of Pd (by 2.1%). The origin of the Pd lattice expansion is discussed and diffusion of Ce species into the Pd lattice seems to be the most probable one. HTR caused also phase transformations in the ceria support. At 973 K and 1100 K, whole CeO2 was transformed into oxygen deficient CeOx phase exhibiting the same or similar structure but with expanded lattice parameter (by 2.8%). At 1180 K most ceria was transformed into hexagonal A-Ce23. The CeOx phase appeared to be stable in hydrogen and in vacuum at room temperature, but upon exposure to air at room temperature it rapidly reoxidised to CeO2. Ce2O3 also reoxidised to CeO2 but much slower. Another consequence of HTR at or above 773 K was formation of pits in CeO2 crystallites, mainly on (112)-type crystal faces. The pits (1–10 nm) exhibited well defined walls parallel to CeO2 lattice fringes and they could possibly constitute nucleation sites for strongly bonded, epitaxial oriented Pd particles.

Nanocrystalline rare earth silicates: structure and properties

Formation of nanocrystalline rare earth (RE) silicates (RE = Y, Nd) inside or at the surface of amorphous SiO 2 matrix upon heal treatment in air was studied by transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). RE-doped SiO 2 samples were prepared by the sol-gel or impregnation method. The structure evolution of the silicate phase depends on the preparation method and RE 2 O 3 :SiO 2 molar ratio. For a Nd 2 O 3 :SiO 2 molar ratio below 1:2, the formation of a superficial silicate, similar to Ce 6 [Si 4 O 13 ][SiO 4 ] 2 reported recently, was observed in the temperature range 900-1100 C.

Structural defects in LaMnO3 phase studied by neutron diffraction

Abstract The crystal structure of air-annealed lanthanum manganite of three different La/Mn ratios (0.91:1, 1:1 and 1:0.9) was refined based on neutron powder diffraction data. Rhombohedral (space group R 3 c ) and orthorhombic (space group Pnma) structure models were taken from the literature available as starting points for the crystal structure analysis. In the course of the Rietveld refinement, several models of different site occupancy were tested. Selection of the most probable models has been based on the reliability factors obtained and on the results of sample density measurements. This selection appeared to be successful for the sample of nominal composition La0.91MnO2.961 (La/Mn=0.91:1) quenched from 1270 K. The resulting distribution model, i.e. [La0.922Mn0.013]MnO3, is characterised by an excess of 0.013 Mn-ions/formula located on the La-site. The type of structural defects existing in samples with nominal compositions of LaMnO3.250 (La/Mn=1:1) quenched from 830 K, and LaMn0.9O3.062 (La/Mn=1:0.9) quenched from 1070 K, both characterised by a supplementary cationic deficiency accompanied by a fraction of excess oxygen, remain undetermined due to large uncertainties in the precise location of the excess oxygen.

Crystal structure of R2Ni2Pb (R=Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) compounds

Abstract The crystal structures of the R 2 Ni 2 Pb (R=Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) compounds were determined using X-ray powder diffraction. The investigated compounds crystallize with Mn 2 AlB 2 structure type (space group Cmmm , Pearson code oC10 ).

X-ray investigation of Cd65Hg35 and Cd55Hg45 single crystals

Abstract X-ray investigations of the crystalline structure and thermal expansion of Cd 65 Hg 35 and Cd 55 Hg 45 crystals within and outside the order-disorder region are reported. Anisotropie temperature factors have been determined. The relative dependences of the intensities of certain reflections on temperature, the broadening of the reflections in the order-disorder range and the dependence of the lattice parameters on temperature as well as on the cooling rates were measured. It is concluded that the crystals are built up of coarse domains of different compositions. It is suggested that such a domain structure is responsible for the diffusion of the ordering and of the critical temperature of superconductivity.

Crystal structure of R6Co2+xPb1-y (R=Y, Gd, Tb, Dy, Ho, Er, Tm, Lu) and R6Ni2+xPb1-y (R=Tb, Dy, Ho, Er, Tm, Lu) compounds

Abstract The crystal structures of R 6 Co 2+ x Pb 1− y (R=Y, Gd, Tb, Dy, Ho, Er, Tm, Lu) and R 6 Ni 2+ x Pb 1− y (R=Tb, Dy, Ho, Er, Tm, Lu) compounds were investigated using X-ray powder diffraction. The investigated compounds crystallize in the Ho 6 Co 2 Ga structure type (space group Immm , Pearson code oI36 ).

Crystal structure of RNiPb (R=Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) compounds

The crystal structures of the RNiPb (R=Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) compounds were determined using X-ray powder diffraction. The investigated compounds crystallize with a TiNiSi structure type (space group: Pnma, Pearson code: oP12).

Effect of oxygen atom disordering on Tc OF YBa2Cu3O7 − x

Abstract The effects of oxygen deficiency and temperature-induced oxygen atom disordering on the critical temperature (Tc) of YBa2Cu3O7 − x are presented. Two possible influences are proposed and discussed to explain an extraordinary behaviour of Tc, observed in the oxygen-deficient region of the phase. The first influence is the effect of oxygen disordering on the b-axis Cu-O chains, which results in a state characteristic of a set of mutually perpendicular and inequivalently occupied Cu-O chains, which is possibly more favourable for superconductivity. The second is the influence of the Cu-O planes, which exist between the yttrium and the barium-ion layers, the contribution of which on Tc may exceed that of the Cu-O chains and, starting at a certain oxygen index, it may become dominating. Estimates on the upper value of Tc for 1:2:3 type compounds (165 K) and on the limit of superconductivity in the YBa2Cu3O7 − x phase have been made. They give 6.23, if determined by the oxygen index, or 3 × 10−3 if determined by the orthorombic distortion parameter p.

X-ray investigations of crystallization and thermal expansion of AuSn4, PdSn4 and PtSn4

Abstract Single crystals of AuSn4 were grown by cooling the molten droplets of the stoichiometric alloy, whereas single crystals of PdSn4 and PtSn4 were grown by fast cooling a molten mixture of the pure metals with an excess of tin. Conditions for superstructure formation in AuSn4 were investigated. In PdSn4 and PtSn4 superstructure was not observed. Lattice parameters were measured as a function of temperature and thermal expansion coefficients were calculated. The results show the different behaviour of AuSn4 in comparison with that of the other two compounds. The frequent appearance of stacking faults and the possibility of a superstructure in AuSn4 seem to account for the discrepancies in the reported superconducting critical temperatures.